U.S. patent application number 14/388261 was filed with the patent office on 2015-02-19 for optical devices, systems and methods.
The applicant listed for this patent is UNIVERSITY OF SOUTHAMPTON. Invention is credited to Kevin Francis MacDonald, David John Richardson, Jianfa Zhang, Nikolay Ivanovich Zheludev.
Application Number | 20150049377 14/388261 |
Document ID | / |
Family ID | 46087134 |
Filed Date | 2015-02-19 |
United States Patent
Application |
20150049377 |
Kind Code |
A1 |
Zheludev; Nikolay Ivanovich ;
et al. |
February 19, 2015 |
OPTICAL DEVICES, SYSTEMS AND METHODS
Abstract
First and second coherent light beams of the same wavelength are
propagated in opposite directions to interact on a sub-wavelength
thickness metallic metamaterial layer which is structured with a
periodicity such that there is a resonance matched to the
wavelength of the coherent beams. The first beam is then able to
modulate the intensity of the second beam by modulating the phase
and/or intensity of the first beam. The interference of the
counter- propagating beams can eliminate or substantially reduce
Joule loss of light energy in the metamaterial layer or, on the
contrary, can lead to a near total absorption of light, depending
on the mutual phase and/or intensity of the interacting beams. A
modulation is thus provided without using a non-linear effect.
Inventors: |
Zheludev; Nikolay Ivanovich;
(Southampton, GB) ; MacDonald; Kevin Francis;
(Southampton, GB) ; Zhang; Jianfa; (Changsha,
CN) ; Richardson; David John; (Southampton,
GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
UNIVERSITY OF SOUTHAMPTON |
Southampon |
|
GB |
|
|
Family ID: |
46087134 |
Appl. No.: |
14/388261 |
Filed: |
March 12, 2013 |
PCT Filed: |
March 12, 2013 |
PCT NO: |
PCT/GB2013/050603 |
371 Date: |
September 26, 2014 |
Current U.S.
Class: |
359/244 |
Current CPC
Class: |
G02B 5/008 20130101;
G02B 1/002 20130101; G02F 2203/10 20130101; G02F 1/0126 20130101;
G02F 2203/15 20130101; G02F 3/00 20130101; G02F 2202/36 20130101;
G02F 2202/30 20130101 |
Class at
Publication: |
359/244 |
International
Class: |
G02F 1/01 20060101
G02F001/01; G02B 5/00 20060101 G02B005/00; G02B 1/00 20060101
G02B001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 26, 2012 |
GB |
1205293.2 |
Claims
1. A device for processing light with light, comprising: a first
input for receiving a signal beam of coherent light at a
wavelength; a second input for receiving a control beam of coherent
light having the wavelength; a metamaterial element having a
thickness and lateral structure having a size scale substantially
smaller than the wavelength of the beams and arranged to receive
the control beam and the signal beam on opposite sides thereof; a
phase and intensity adjuster operable to set the mutual phase of
the signal and control beams such that a standing wave is formed
having a node or an antinode coinciding with the metamaterial
element; and an output for transmitting a component of the signal
beam after transmission through the metamaterial element.
2. The device of claim 1, further comprising a control laser source
for providing the control beam to the second input.
3. The device of claim 1, further comprising a signal laser source
for providing the signal beam to the first input.
4. The device of claim 1, further comprising a laser source for
providing both the signal beam and the control beam to the first
and second inputs respectively.
5. The device of claim 1, further comprising a sensor for detecting
intensity of the component of the signal beam transmitted through
the output.
6. The device of claim 5, wherein the sensor is connected to the
phase and intensity adjuster to set the mutual phase responsive to
detected intensity.
7. The device of claim 1, wherein the phase and intensity adjuster
includes a phase modulator operable to vary the phase of the
control beam incident on the metamaterial element, so as to switch
between a node and an antinode of the interference pattern
coinciding with the metamaterial element to effect a modulation of
the signal beam's transmission through the metamaterial
element.
8. The device of claim 1, wherein the phase and intensity adjuster
includes an intensity modulator operable to vary the intensity of
the control beam incident on the metamaterial element between first
and second intensities to effect a modulation of the signal beam's
transmission through the metamaterial element.
9. The device of claim 1, wherein the lateral structure of the
metamaterial element has a period which provides the metamaterial
element with a resonance at the wavelength.
10. The device of claim 9, wherein the resonance is a plasmon
resonance.
11. The device of claim 1, wherein the output is also arranged to
transmit a component of the control beam after transmission through
the metamaterial element.
12. The device of claim 1, wherein the thickness and lateral
structure of the metamaterial element are dimensioned such that the
transmitted component of the signal beam in the presence of said
control beam when a node of the interference pattern is coincident
with the metamaterial element has an intensity greater than in the
absence of said control beam.
13. The device of claim 1, wherein the thickness and lateral
structure of the metamaterial element are dimensioned such that the
transmitted component of the signal beam in the presence of said
control beam when a node of the interference pattern is coincident
with the metamaterial element has at least 70, 80, 90, 95 or 98% of
the intensity of the signal beam incident on the metamaterial
element.
14. The device of claim 1, wherein the metamaterial element is
arranged embedded in, or on the end face of, a waveguide transverse
to the waveguide channel.
15. A method of processing light with light comprising: providing a
signal beam of coherent light at a wavelength; providing a control
beam of coherent light having the wavelength; providing a
metamaterial element having a thickness substantially smaller than
the wavelength of the beams and structured laterally on a size
scale substantially smaller than the wavelength of the beams;
directing the control beam and the signal beam to be incident on
the metamaterial element in opposite directions; and setting the
mutual phase of the signal and control beams such that a standing
wave is formed having a node or an antinode coinciding with the
metamaterial element, thereby to control transmission of the signal
beam through the metamaterial element.
16. The method of claim 15, wherein the mutual phase is modulated
to alternate between a node and an antinode of the interference
pattern coinciding with the metamaterial element to effect a
modulation of the signal beam's transmission through the
metamaterial element.
17. The method of claim 15, wherein the intensity of the control
beam is modulated to effect a modulation of the signal beam's
transmission through the metamaterial element.
18. The method of claim 15, further comprising: measuring the
intensity of the signal beam after its transmission through the
metamaterial element.
19. The method of claim 15, wherein the lateral structure has a
periodicity matched to the wavelength.
20. The method of claim 15, wherein the thickness and lateral
structure of the metamaterial element are dimensioned such that the
transmitted component of the signal beam in the presence of said
control beam when a node of the interference pattern is coincident
with the metamaterial element has an intensity greater than in the
absence of said control beam.
21. The method of claim 15, wherein the thickness and lateral
structure of the metamaterial element are dimensioned such that the
transmitted component of the signal beam in the presence of said
control beam when a node of the interference pattern is coincident
with the metamaterial element has at least 70, 80, 90, 95 or 98% of
the intensity of the signal beam incident on the metamaterial
element.
22. The method of claim 15, wherein the lateral structure of the
metamaterial element has a period which provides the metamaterial
element with a resonance at the wavelength.
23. The method of claim 22, wherein the resonance is a plasmon
resonance.
Description
BACKGROUND OF THE INVENTION
[0001] The invention relates to devices and related systems and
methods for affecting transmission of a first light beam passing
through a layer from one side through use of a second light beam
incident on the layer from the other side. The invention finds
application in fields such as optical signal processing, optical
computing, optical sensing and optical spectroscopy.
[0002] The common wisdom in optics is that light beams travelling
in different and even opposite directions pass though one another
without mutual disturbance. This is known as the superposition
principle of linear optics.
[0003] In order to allow light signals to interact in such a way
that one light signal can modulate or control another light signal,
a non-linear medium is used in which intense optical fields
provided by lasers interact. Such arrangements allow the
superposition principle to be broken in nonlinear optics.
[0004] However, using non-linear effects in a non-linear medium for
beam interaction typically requires intense laser fields thereby
necessitating high power consumption and significant costs. These
features of non-linear interactions make light-by-light modulation
either unavailable or unsuitable for many applications, such as
data processing, where it could otherwise be very useful.
[0005] Composite and layered structures have attracted recent
interest to provide so-called coherent perfect absorption (CPA),
i.e. to absorb the entirety of an incident laser beam.
[0006] Dutta-Gupta et al, "Controllable coherent perfect absorption
in a composite film" Optics Express, vol. 20 p. 1330-1336 (2012)
describe how a metal/dielectric composite might be used to achieve
coherent perfect absorption (CPA) in a plasmonic metal/dielectric
composite slab of thickness d=5 .mu.m which is illuminated by
coherent light from both sides of wavelength .lamda.=562 nm. The
light wavelength is matched to the plasmon resonance of the slab
which is at around .lamda.=540 nm. The paper suggests tuning the
plasmon resonance of the composite by varying the volume fraction
of the metal.
[0007] Pu et al, "Ultrathin broadband nearly perfect absorber with
symmetrical coherent illumination" Optics Express, vol. 20 p.
2246-2254 (2012) describe how a thin layer of tungsten of thickness
17 nm can be used as a CPA device. A tungsten CPA is expected on
the basis of the metal's bulk dielectric permittivity to have an
operational wavelength range of 800 nm-1500 nm and also have
absorption over a very broad wavelength range, so it is suggested
for use in a solar cell for absorbing sunlight.
SUMMARY OF THE INVENTION
[0008] According to one aspect of the invention, there is provided
a device for processing light with light, comprising: a first input
for receiving a signal beam of coherent light at a wavelength; a
second input for receiving a control beam of coherent light having
the same wavelength; a metamaterial element having a thickness and
lateral structure having a size scale substantially smaller than
the wavelength of the beams and arranged to receive the control
beam and the signal beam on opposite sides thereof; a phase and
intensity adjuster operable to set the mutual phase of the signal
and control beams such that a standing wave is formed having a node
or an antinode coinciding with the metamaterial element; and an
output for transmitting a component of the signal beam after
transmission through the metamaterial element.
[0009] The metamaterial element could be a film or layer structured
on the sub-wavelength scale in a periodic fashion. This may be a
film of metal, metal alloy, conductive oxide, graphene, carbon
nanotubes, fullerenes or semiconductor. The film or layer is
structured in a way to provide enhanced optical absorption on the
said wavelength so for optimal operation the absorption of the film
in one direction is 50 percent, i.e. in practice close to 50
percent, such as to within 50.+-.10%, 50.+-.5%, 50.+-.4%, 50.+-.3%,
50.+-.2% or 50.+-.1%.
[0010] The sub-wavelength thickness of the metamaterial sheet
enables it to become a `perfect` transmitter when the mutual phase
of the signal and control beams has a node in the plane of the
sheet. Without satisfying that requirement, i.e. with a `thick`
film having a thickness of around half a wavelength or more, full
or near-full transmission cannot be achieved and the modulation
capability of the metamaterial layer will be limited. Moreover, the
lateral structure in the plane of the metamaterial sheet allows a
sufficiently strong resonant absorption to be provided at the
design wavelength by providing a periodic metastructure matched to
the design wavelength.
[0011] The device may further comprise a control laser source for
providing the control beam to the first input and/or a signal laser
source, coherent with the first laser source for providing the
signal beam to the second input. Alternatively, a single laser
source can provide both the control beam and the signal beam to the
first and second inputs respectively. A sensor can be incorporated
as part of the device for detecting intensity of the component of
the signal beam transmitted through the output. The sensor can be
connected to the intensity and phase adjuster, e.g. by an
electrical control line, to set the mutual phase and intensity
responsive to detected intensity.
[0012] The adjuster can include a phase modulator. The phase
modulator is operable to vary the phase of the control beam
incident on the metamaterial element, so as to switch between a
node and an antinode of the standing wave, i.e. interference
pattern, coinciding with the metamaterial element, thereby to
effect a modulation of the signal beam's transmission through the
metamaterial element. When a phase modulator is provided, it can be
operated to vary the phase of the control beam incident on the
metamaterial element in amounts of n.lamda./2, where n=1, 3, 5
etc., so as to switch between a node and an antinode being
coincident with the metamaterial element.
[0013] An intensity modulator can be provided to vary the intensity
of the control beam incident on the metamaterial element between
first and second intensities to effect a modulation of the signal
beam's transmission through the metamaterial element. For example,
if the phase adjuster maintains the mutual phase of the signal and
control beams such that their interference pattern has a node at
the metamaterial element, then intensity modulation of the control
beam, e.g. to selectively switch off the control beam, will serve
to intensity modulate the transmitted signal beam between perfect
transmission (with the control beam) and partial transmission
(without the control beam). When an intensity modulator is
provided, it is preferably operable to vary the intensity of the
control beam incident on the metamaterial element between first and
second intensities, wherein the first intensity is at least 10
times smaller than the second intensity, and further preferably
substantially zero.
[0014] The metamaterial element can have metastructure of a
suitable periodicity to provide a resonance that has at least a
substantial component at the wavelength of the control and signal
beams, i.e. the resonance is matched to the operating wavelength of
the device. This resonance may have plasmonic nature or could be
related to other resonance excitations of the metamaterial
structure.
[0015] Examples of materials that could provide a suitable layer
for supporting plasmons are: gold, silver, aluminium, copper,
alkali metals, intermetallics (silicides, germanides, borides,
nitrides, oxides, and metallic alloys including titanium nitride,
tungsten/tantalum silicide or germanide,
vanadium/titanium/aluminium oxides), transparent conductive oxides
(e.g. indium tin oxide, aluminium/gallium-doped zinc oxide, silicon
carbide, gallium arsenide), graphene, and semiconductors.
[0016] In some embodiments, the output is also arranged to transmit
a component of the control beam after transmission through the
metamaterial element. For example the transmitted components of the
signal and control beams can be combined and output together.
[0017] The thickness and lateral structure of the metamaterial
element can be dimensioned such that the transmitted component of
the signal beam in the presence of said control beam when a node of
the interference pattern is coincident with the metannaterial
element has an intensity greater than in the absence of said
control beam.
[0018] The thickness and lateral structure of the metamaterial
element can be dimensioned such that the transmitted component of
the signal beam in the presence of said control beam when a node of
the interference pattern is coincident with the metamaterial
element has at least 70, 80, 90, 95 or 98% of the intensity of the
signal beam incident on the metamaterial element.
[0019] The metamaterial element can be embedded in, or on the end
face of, a waveguide transverse to the waveguide channel. The
waveguide can be an optical fibre. In a conventional fibre, the
channel will be the single core of the optical fibre. However, the
channel may be the cladding of a cladding pumped fibre or multiple
cores of a multicore fibre. The waveguide may also be a planar
waveguide, e.g. semiconductor or lithium niobate or related
material.
[0020] The metamaterial element can be a free-standing element or
can be attached to a substrate. The substrate in most embodiments
will be transparent, but in some embodiments could be partially
absorbing film to increase overall absorption.
[0021] According to another aspect of the invention, there is
provided a method of processing light with light comprising:
providing a signal beam of coherent light at a wavelength;
providing a control beam of coherent light having the wavelength;
providing a metamaterial element having a thickness substantially
smaller than the wavelength of the beams and structured laterally
on a size scale substantially smaller than the wavelength of the
beams; directing the control beam and the signal beam to be
incident on the metamaterial element in opposite directions; and
setting the mutual phase of the signal and control beams such that
a standing wave is formed having a node or an antinode coinciding
with the metamaterial element, thereby to control transmission of
the signal beam through the metamaterial element.
[0022] The method may further comprise: measuring the intensity of
the signal beam after its transmission through the metamaterial
element.
[0023] The metamaterial element can be made from a structured metal
layer supported by a substrate. The substrate will typically be
made of a material that is substantially transparent in the
operating wavelength range of the metamaterial material. In other
cases, the periodically structured layer is self supporting, i.e.
there is no substrate, at least not over the active area exposed to
the control and signal beams. The lateral metastructuring is
preferably periodic in two-dimensions (2D). Three-dimensional (3D)
or one-dimensional (1D) periodicity could also be used. In the case
of 2D or 3D structuring, the period in each of the two- or
three-dimensions is preferably equal. The layer can be fabricated
as a metamaterial having in-plane structure of a dimension less
than half the operating wavelength, or less than half of the
minimum operating wavelength in the case the device has a range of
operating wavelengths. The metamaterial layer may also have
out-of-plane sub-wavelength structure that satisfies the same
dimensional criteria as the in-plane structure. The in-plane and
out-of-plane structure is preferably periodic. The in-plane
structure may be periodic in one direction or in two non-parallel
directions, for example two orthogonal directions.
[0024] The device based on a metamaterial film sheet can be made to
operate at any desired wavelength across the visible (e.g. 400
nm-700 nm) to near-infrared wavelength (e.g. 700 nm to 2.5
micrometres) range by choosing the structuring of the metamaterial
accordingly.
[0025] Applications of the present devices include ultrafast
pulse-recovery devices, coherence filters and THz-bandwidth
light-by-light modulators. Since the present devices do not require
non-linear media or intense laser fields, they can operate at
extremely low power levels.
[0026] The metamaterial element could be a film or layer structured
on the sub-wavelength scale a periodic fashion. The film or layer
may be made of metal, metal alloy, conductive oxide, graphene,
carbon nanotubes, fullerenes or semiconductor. The film or layer
can be structured in a way to provide enhanced optical absorption
at the design wavelength, so for optimal operation the absorption
of the film in one direction is 50 percent.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] The invention is now described by way of example only with
reference to the following drawings.
[0028] FIGS. 1a and 1b are schematic drawings illustrating the
principle of light-by-light modulation devices embodying the
invention.
[0029] FIG. 2a shows an example of a metastructured plasmonic metal
film providing a metamaterial, together with an enlarged image of a
single unit cell.
[0030] FIG. 2b is scanning electron micrograph of a metal
metastructure pattern formed on the end face of an optical
fibre.
[0031] FIG. 3 illustrates examples of some possible metastructure
unit cell geometries.
[0032] FIG. 4 shows a system example for realising light-by-light
modulation using the scheme of FIGS. 1a and 1b.
[0033] FIG. 5a is a scanning electron micrograph of a portion of
the metastructure element used in the system example of FIG. 4.
[0034] FIGS. 5b and 5c respectively show numerically simulated and
experimentally measured graphs of transmission T, reflection R and
absorption A spectra for the metamaterial element used in the
system of FIG. 4.
[0035] FIG. 6a shows how the transmitted intensities I.sub.S of the
signal beam A and the control beam B change in response to change
of phase .phi. of the control beam B in the system of FIG. 4.
[0036] FIG. 6b shows how total intensity I.sub.T of light
transmitted through the metamaterial depends on the mutual phase
.phi. of the signal beam A and control beam B in the system of FIG.
4.
[0037] FIG. 6c shows modulation of the output intensity of the
signal beam resulting from modulation of the control beam in the
system of FIG. 4.
[0038] FIG. 7 shows the simulated absorption performance over a
range of wavelengths of a free-standing (substrate free) 50 nm
thick gold metamaterial film with an absorption line designed for
the telecom band centred at 1550 nm.
[0039] FIG. 8a shows transmission T, reflection R and absorption A
spectra for the metannaterial film. FIG. 8b shows transmission S
and absorption A profiles at the 1550 nm absorption resonance
wavelength. FIG. 8c corresponds to FIG. 8b but at a non-resonant
wavelength of 1350 nm. FIG. 8d, for comparison, shows transmission
S and absorption A spectra for an unstructured 50 nm thick gold
film.
[0040] FIG. 9a illustrates a pulse restoration /clock recovery
device.
[0041] FIG. 9b illustrates a coherency filter.
[0042] FIG. 9c illustrates an optical gate.
[0043] FIG. 10a illustrates a pulse picker/selector device.
[0044] FIG. 10b illustrates a frequency selector device.
[0045] FIG. 10c illustrates a spatial mode selector device.
[0046] FIG. 11 is a schematic system of an optical fibre
transmission system incorporating the pulse recovery device of FIG.
9a.
[0047] FIGS. 12a, 12b and 12c are graphs showing the spectral
density of the distorted signal, clock signal and improved signal
respectively for the system of FIG. 11.
DETAILED DESCRIPTION
[0048] In the following, we describe how first and second coherent
light beams of arbitrarily low intensity are propagated in opposite
directions to interact on a sub-wavelength thickness plasmonic
sheet or layer such that the first beam is able to modulate the
intensity of the second beam by modulating the phase and/or
intensity of the first beam. Moreover, we show experimentally and
illustrate by computer modelling that interference of
counter-propagating beams can eliminate or substantially reduce
plasmonic Joule loss of light energy in the plasmonic layer or, on
the contrary, can lead to a near total absorption of light,
depending on the mutual phase and/or intensity of the interacting
beams. The coherent light beams can be of arbitrarily low
intensity, since the effect being exploited is not a non-linear
one.
[0049] FIGS. 1a and 1b are schematic drawings illustrating the
operating principles of a device example to provide light-by-light
modulation. The device operates using first and second coherent
light beams A and B of the same wavelength. A thin planar
light-absorbing plasmonic metamaterial element 2 is arranged to
receive the light beams A and B from either side. The metamaterial
element 2 is ideally an infinitely thin sheet compared with the
intended operating wavelength of the device. In practice, the
thickness of the metamaterial element needs to be considerably less
than the intended operating wavelength of the device, for example
less than .lamda./5, .lamda./6, .lamda./7, .lamda./8, .lamda./9 or
.lamda./10. In both FIGS. 1a and 1b, beam A is incident on the
plasmonic metamaterial element 2 from left to right, and beam B is
incident on the metamaterial element 2 from right to left. Beams A
and B are thus incident on the metamaterial element 2 from opposite
directions. FIGS. 1a and 1b show the two extreme cases of
interference of the two light beams A and B at the metamaterial
element 2.
[0050] In the first case, as shown by FIG. 1a, beams A and B
interfere such that a standing wave pattern 3 is formed with a
zero-field node 4 at the position of the metamaterial element 2. As
the metamaterial element 2 is much thinner than the wavelength of
the light, its interaction with the electromagnetic field at the
field minimum is negligible. The metamaterial element will
therefore act as if it were not present, i.e. as if it were
transparent to each of the beams.
[0051] In the second case, as shown by FIG. 1b, the metamaterial
element is at a standing wave field maximum of the superposed
fields from A and B, i.e. at an antinode 6. The metamaterial
element in this case strongly disturbs the wave. Absorption becomes
very efficient and the metamaterial element 2 is opaque to both
beams, completely blocking transmission of the light in both
directions, at least in the ideal case. The broken lines and broken
arrows in FIG. 1b represent absence of the light waves, i.e. the
absence of beams A and B as a result of the beam having been
absorbed by the metamaterial element 2.
[0052] Altering the phase or intensity of one of the beams will
disturb the interference pattern and thus will change the
absorption of the other beam. For instance, if the metamaterial
element 2 is placed at a node 4 of the standing wave, blocking of
beam B will lead to the immediate increase of loss and decrease of
intensity of the transmitted beam A. In another example, if the
metamaterial element is placed at an antinode 6 of the standing
wave, blocking beam B will lead to the decrease of loss and
increase of intensity of the transmitted beam A.
[0053] In one mode of operation, the device thus allows the
intensity of beam A transmitted through the material element 2 to
be changed by manipulating the intensity of beam B, e.g. by
blocking beam B or switching off beam B. In another mode of
operation, the device thus allows the intensity of beam A
transmitted through the metamaterial element 2 to be changed by
manipulating the phase position of the metamaterial element 2 in
the beams A and B. e.g. by altering the phase of one or both of the
beams A and B or by moving the metamaterial element 2 in the beam
propagation direction.
[0054] To optimize the modulation efficiency, the metamaterial
element 2 should ideally absorb half the energy of a single one of
the beams A or B passing through it. In this case, 100%
light-by-light modulation can be achieved when beam A is modulated
by controlling the phase of beam B. Also, 50% modulation can be
achieved when beam A is modulated by controlling the intensity of
beam B, since if the intensity of beam B is reduced to zero, the
metamaterial element is illuminated by the beam A and, as already
stated, the metamaterial element is designed to absorb half the
energy of a single beam passing through it. Moreover, when the
intensity of both beams are equal and the metamaterial element is
placed in an antinode 6, all light entering the metamaterial
element will be absorbed, while if the metamaterial element is
placed in a node 4, no Joule losses will take place and all light
entering the metamaterial element will be transmitted.
[0055] The metamaterial element 2 is a very thin film made from a
nanostructured metamaterial. The film is thin compared to the
wavelength of the incident light. In particular in the visible and
near-infrared part of the spectrum, this allows absorption of a
single beam approaching 50% at a particular absorption resonance
wavelength to be achieved.
[0056] Nanostructured plasmonic metamaterials are a type of
artificial medium structured on a size scale smaller than the
wavelength of an external stimulus, wherein the sub-wavelength
structure has a sufficiently small periodicity to avoid
diffraction. The plasmonic material will typically be a metal, but
may also be a non-metal capable of supporting a surface plasmon.
The metal is preferably gold, silver, aluminium, copper, or an
alloy including one or more of these metals and a further metal or
metals, or an alloy consisting only of two or more of these metals.
A periodic structure of the metal is known as a metastructure or
nanostructure, and may be produced by, for example, direct milling
with a focussed ion beam or electron beam lithography or
photolithography or nano-imprint or self-assembly of
nanostructure.
[0057] FIG. 2a shows an example of a metastructured metal film 8
that could be used as a plasmonic metamaterial, together with an
enlarged image of the unit cell 10 of the periodic metastructure
pattern. In this example, the metastructure pattern is an
asymmetric split-ring pattern. The metamaterial pattern has a
period of 425 nm in both orthogonal in plan directions x and y, and
has an absorption resonance wavelength of approximately 1500 nm.
The shape and size of the unit cell of the metastructure alters the
resonance absorption wavelength.
[0058] FIG. 2b is scanning electron micrograph of a metal
metastructure pattern 8 formed on the end face 7 of an optical
fibre covering the core 9. The fibre coating 11 has been stripped
away to leave an end portion of the bare fibre 9 free standing to
assist the imaging. The detailed inset shows an enlarged view of
the area of a 1 .mu.m square portion of the metal metastructure. In
this example, the bare fibre, and hence the cladding, has a
diameter of approximately 125 .mu.m and the core diameter is 9
.mu.m. The size of the metal metastructure is 50 .mu.m square
covering the core and adjacent parts of the cladding. The
illustrated inset shows a 1 .mu.m square portion of the metal
metastructure from which it can be seen that the metastructure
period is approximately 200 nm.
[0059] Further, the metastructure may be embedded in a continuous
fibre structure by fusing a further fibre to the end of the fibre
with the metal metastructure.
[0060] In other embodiments, the metamaterial may be formed on and
supported by a surface of another type of waveguide, such as a
planar waveguide. The substrate may be made of any conventional
material such as a glass, semiconductor, crystal or lithium niobate
or related compounds.
[0061] The designs most closely considered to date use a specific
example of an asymmetric split-ring metamaterial pattern in gold.
Other suitable metals include silver, aluminium or copper. In
principle, any surface plasmonic material should work which will
include other metals and some non-metals, such as transparent
conductive oxides (for infrared applications) graphene,
semiconductor carbon nanotubes and semiconductors. A suitable
conductive oxide is indium tin oxide (ITO). Suitable semiconductors
are silicon carbide and gallium arsenide. The device can also be
exemplified with a wide range of periodic metamaterial pattern
geometries including circular rings, oval rings, fishnet grids and
so forth. FIG. 3 illustrates examples of some possible pattern
geometries. Positive and negative examples of each pattern can be
used, i.e. the structure may be formed by absence of metal (e.g.
milling material out of a complete sheet) or presence of metal
(e.g. selective deposition of line-like structures). Most current
metastructures are based on planar or two-dimensional (2D)
patterning. As technology progresses it is expected that techniques
for fabricating three-dimensional (3D) metastructures will be
developed, and the metamaterial element can also be used with such
3D metastructures.
[0062] Theoretically 50% single beam absorption is the maximum
absorption that can be achieved in a thin film, including a
metamaterial thin film, as now explained. At normal incidence, the
reflection coefficient r and transmission coefficient t of the thin
film in a symmetric environment are related to each other as
t=1.+-.r where the upper and lower signs are for s-polarized and
p-polarized light respectively. The maximum absorption is then
given by A=1-|r|.sup.2-|1.+-.r|.sup.2, which is limited to 50%
(corresponding to r=1/2) [see Thongrattanasiri, Koppens and Garcia
de Abajo, "Complete Optical Absorption in Periodically Patterned
Graphene" Phys. Rev. Lett. 108, 047401 (2012)]. This value is
increased when the film is thick (relative to the optical
wavelength) or the environment is asymmetric, such as if the
metamaterial film is fabricated on a dielectric substrate,
resulting in different reflection and absorption for light incident
from different sides.
[0063] FIG. 4 shows an example of an experimental system 12 for
light-by-light modulation using the scheme of FIGS. 1a and 1b. A
linearly polarized beam of light from a HeNe laser (outputting
laser light of wavelength .lamda.=632.8 nm) is split by a
beam-splitter BS1 into two beams A and B which are adjusted to
equal intensities by an attenuator ATT. The two beams A and B
constitute the "signal" and "control" beams respectively, and are
guided by a number of plane mirrors M. The beams are directed onto
the metamaterial element 2 comprising the plasmonic metamaterial
from opposite directions by parabolic mirrors PM. The phase of
control beam B can be changed by a variable optical delay VOD and
its intensity can be modulated by a modulator MOD to provide
modulation in phase and amplitude as desired, thereby to modulate
signal beam A.
[0064] The intensity of the beams transmitted through the
metamaterial element 2 is monitored by the photo detector DET. A
control shutter CS for shuttering the control beam B and a signal
shutter SS for shuttering the signal beam A allow the photo
detector DET to operate in two different regimes. In the first
regime, both shutters CS and SS are open and therefore the photo
detector DET registers the combined intensity of both beams (the
difference of total travel distances for the signal and control
beams to the detector is much longer the coherence length of the
laser radiation so the beams do not interfere on the detector). In
the second regime, one of the shutters CS and SS is closed and the
other is open, so the photo detector DET only detects the intensity
of the non-shuttered beam.
[0065] In this particular example, the metamaterial element 2
comprises a metamaterial with an asymmetric split-ring
metastructure pattern (the pattern being similar to that shown in
FIG. 2) fabricated in a 50 nm thick gold film (corresponding to
approximately .lamda./13 thickness, given the 632.8 nm wavelength
of the laser light). The film is supported by a silica substrate of
approximately 170 .mu.m thickness and surface roughness of less
than 0.5 nm. The 50 nm gold is deposited on the silica substrate
using low pressure 10.sup.-7 mbar thermal evaporation at a
deposition rate of 0.05 nm/s. The metastructure is fabricated by
direct milling with a focussed ion beam. The metastructure of the
film supports a plasmonic Fano-type plasmonic mode of excitation
that leads to a strong resonant absorption for y-polarized light
(the y-direction with respect to the metastructure pattern is shown
in FIG. 2). It is important that the size of the unit cell of the
metamaterial is small enough that it does not diffract light at the
laser wavelength. For this particular example, a metamaterial unit
cell size of 250 nm.times.250 nm has been chosen, i.e.
significantly less than the diffraction limit of .lamda./2.
[0066] In the system of FIG. 4, the phase tuning or modulation is
in arm B. It would instead be possible to modulate the phase in arm
A, or in both arms. What is important is the mutual phase of beams
A and B at the plane of the metastructure element 2.
[0067] A control line 13 may be provided to connect the detector
and the phase controller. The mutual phase can then be controlled
in a feedback loop based on the intensity of the output signal
received by the detector. This can be done once on set up to lock
the phase relationship to the desired state--typically to set a
node or antinode on the metamaterial element--or maintained a
particular mutual phase during use in an ongoing manner. Another
mode of operation would be to modulate the phase during use between
node and antinode positions on the metamaterial element in discrete
changes which may be with or without assistance from the control
signal conveyed by the control line.
[0068] FIG. 5a is a scanning electron micrograph of a portion of
the metastructure element used in the system example of FIG. 4.
[0069] FIGS. 5b and 5c respectively show numerically simulated and
experimentally measured graphs of transmission, reflection R and
absorption A spectra for the metamaterial element used in the
system of FIG. 4. The light is incident on the metamaterial from
the air side (that is, the non-substrate side) at normal incidence
with y-polarisation. It can be seen that the experimental results
agree well with the simulations. The experimental spectra of FIG.
5c were obtained using a microspectrophotometer by CRAIC
technology.
[0070] FIG. 6a shows how the transmitted intensities I.sub.S of the
signal beam A and the control beam B change in response to changing
the phase .phi. of the control beam B in the experimental
arrangement 12. Here, the phase .phi. of the control beam B is
changed by the variable optical delay VOD in the B arm of the
experimental arrangement 12. One can see that, upon changing the
phase .phi. of the control beam B, the metamaterial element 2 is
moved from a node of the standing wave (.phi.=.pi., 3.pi.) to an
anti-node (.phi.=0, 2.pi.) and the transmitted intensity of the
signal beam A passing through the metamaterial element is modulated
between 115% and 10% of the incident intensity. At the same time,
the transmitted intensity of the control beam B passing through the
metamaterial element 2 is modulated between 64% and 15%. The signal
beam modulation A MOD and control beam modulation B MOD are shown
in FIG. 6a.
[0071] For an ideal, free-standing, zero-thickness 50% absorber one
would see the signal beam A modulated between 0% and its full 100%
incident intensity level. The somewhat different limits between
which experimental modulation is observed are explained by a number
of factors: Firstly, the sample's absorption level at the laser
wavelength is not exactly 50%. Indeed, due to the presence of a
substrate and to fabrication-related asymmetry/imperfection of the
slots milled into the gold film, it shows differing levels of
absorption (34% and 57%) for the two opposing propagation
directions; Second, although the metamaterial is very thin it does
have a finite thickness of .lamda./13; And finally, the laser
source is not perfectly coherent--its emission includes an
incoherent luminescence component.
[0072] FIG. 6b shows how the total output intensity I.sub.T of
light transmitted through the metamaterial (that is, the
transmitted intensity of the signal beam A plus the transmitted
intensity of the control beam B) depends on the mutual phase .phi.
of the signal beam A and control beam B. It can be seen that nearly
perfect absorption can be achieved when the mutual phase of the
incident beams A and B is set to (.phi.=0, 2.pi.).
[0073] For comparison, the output intensity variations for a simple
unstructured gold film are also shown, in other words a control
film which is of the same material and thickness as the
metastructure film, but does not have any metastructuring, and
hence no resonance at the beam wavelength. It can be seen that the
phase evolution of the modulation is the same, but the amplitude of
the modulation much weaker. This is a specific example illustrating
the more general point that an equivalent unstructured thin metal
film will have lower absorption than a metastructured counterpart,
so in many cases it may be impossible to achieve the desired 50%
absorption while at the same time meeting the device requirement
that the film thickness is much smaller than the wavelength of the
light. Moreover, use of a periodic metastructure allows design
freedom to select any desired resonance wavelength across a broad
range of the visible and near infrared spectrum by selecting an
appropriate period, e.g. a period matched to a particular laser
output frequency and/or a particular optimum frequency for long
haul transmission through a telecoms fibre, such as the frequency
of minimum dispersion or minimum absorption.
[0074] FIG. 6c shows modulation of the combined output intensity
resulting from modulation of the control beam in the system of FIG.
4. More specifically, the graph shows the modulation of total
output intensity resulting from modulation of the control beam's
intensity in the time domain. When the control beam is blocked,
only the signal wave is present at the metamaterial and the
standing wave regime of light-metamaterial interaction is replaced
by the traveling wave regime: In this example the metamaterial is
initially located at a node of the standing wave where absorption
is minimal (combined output intensity=95% of input); interruption
of the control beam `switches on` signal beam absorption and
output, i.e. transmission, drops to approximately 20% of the input
level. This proof-of-principle demonstration employs a mechanical
chopper running at only 1.07 kHz. However, since the cross-beam
modulation bandwidth will be limited only by the width of the
resonant absorption peak, the inherent bandwidth of the process is
likely to be in the THz range.
[0075] The above example shows controlling of light-with-light by
absorption in a plasmonic metamaterial, which is achieved through
adjustment of the mutual phase of signal and control beams incident
on the plasmonic metamaterial.
[0076] In the example above, a metamaterial of .lamda./13 thickness
was used. A different thickness could also have been used. However,
to maintain the light-by-light modulation of the present invention,
the metamaterial thickness should be kept sufficiently small
compared to the wavelength of the light that is to be modulated.
This is to ensure that the entirety of the metamaterial thickness
can be kept within the vicinity of the standing wave node or
antinode, as appropriate.
[0077] FIG. 7 shows the simulated absorption performance over a
range of wavelengths of a free-standing (substrate free) 50 nm
thick gold metamaterial film with an absorption line designed for
the telecom band centred at 1550 nm. This illustrates that the
cross-beam modulation bandwidth is only limited by the width of the
resonant absorption peak of the metamaterial and thus potentially
allows for a THz bandwidth. Again, the metamaterial pattern is the
asymmetric split-ring type, as can be seen from inset of the graph
showing the metamaterial unit cell 14. Curve (a) shows the
absorption A.sub.S for a single beam incident on the metamaterial
and curve (b) shows the total output intensity I.sub.T when both a
signal beam A and control beam B are incident on the metannaterial
so that an antinode 6 is present at the metamaterial film.
[0078] FIG. 8a shows transmission T, reflection R and absorption A
spectra for the metamaterial. It can be seen that there is a
maximum absorption of 50.18% at the 1550 nm absorption resonance
wavelength. It is slightly higher than the ideal 50% due to the
finite thickness of the metamaterial film.
[0079] FIG. 8b shows transmission S and absorption A profiles at
the 1550 nm absorption resonance wavelength. It is seen that near
perfect plasmonic transparency and absorption can be realized at
this resonance wavelength by controlling the relative phase .phi.
of the incident signal and control beams. The broken curves S.sub.1
and S.sub.2 represent the intensity of the two output ports of the
virtual interferometer used in the simulation. These outputs are
analogous to the signal beam and control beam inputs to the photo
detector DET of FIG. 4, in that they measure the intensity of the
transmitted signal beam and control beam separately.
[0080] FIG. 8c shows that at a non-resonant wavelength of 1350 nm,
the metamaterial absorption is much smaller and the transmission is
high. The metamaterial thus behaves like a normal interferometer,
with energy transferring between the two output ports as the
relative phase .phi. of the signal beam and control beam
changes.
[0081] FIG. 8d, for comparison, shows transmission S and absorption
A spectra for unstructured 50 nm thick gold film (this gold film is
therefore not a metamaterial). It can be seen that the transmission
and absorption only varies by around 2% as the relative phase cp is
changed. This is due to the fact that most of the light incident on
unstructured gold is not transmitted or absorbed, but is reflected.
The reflected light is thus detected at the output ports.
[0082] The simulated results of FIGS. 5a, 7 and 8a-8d were obtained
using a fully three dimensional finite element package by COMSOL
Multiphysics. Experimental values of the complex dielectric
parameters for gold were utilised. For producing FIGS. 5a, 7 and
8a-8c, the following parameters were used:
.epsilon..sub.gold=-9.51588-1.12858i for gold permittivity,
.epsilon..sub.silica=2.1316 for silica substrate permittivity (FIG.
5a only) and .epsilon..sub.air=1 for air permittivity. For FIG. 8d
(modelling the unstructured gold film),
.epsilon..sub.gold=-132.024-12.6637i was used for the gold
permittivity.
[0083] The modelling used for the simulations above relies on the
well established data for gold complex conductivity taken from E.
D. Palik, "Handbook of Optical Constants of Solids", Academic
Press, San Diego, 1998. The simulations show that the metamaterial
of FIGS. 7 and 8a will exhibit 50.18% single beam absorption at the
1550 nm absorption resonance wavelength. FIG. 8b shows that in the
case that both the signal beam A and the control beam B are
incident on the metamaterial at the resonant wavelength, the total
absorption can be controlled to be between 0.38% to 99.99%. Also,
the total transmitted intensity can be controlled to be between
0.01% to 99.62%.
[0084] The relatively broad nature of the metamaterial provides for
modulation between 1% and 90% of total intensity levels across the
entire spectral range from 1530 to 1575 nm, corresponding to 5.6
THz bandwidth.
[0085] The example of FIG. 7 illustrates the potential application
for modulating telecom signals, i.e. signal processing. As well as
telecom applications, the high sensitivity of absorption to the
mutual phase of the signal and control beams lends itself for
applications in sensors and laser spectroscopy.
[0086] FIG. 9a illustrates a pulse recovery device 15 as may be
used in a receiver to recover the clock signal from a distorted
signal transmitted over a long haul optical fibre, for example. In
optical data systems, pulses become distorted as they travel
because of dispersion and non-linear interactions, which slows down
data distribution over processing networks. A distorted pulse 16
comprised within the signal beam A can be cleaned up by interacting
with a clock pulse 18 comprised within the control beam B at the
metamaterial element 2. If the phase of the clock pulse 18 is
chosen correctly, spectral components of the distorted pulse 16
that have the same intensity and amplitude as the clock pulse 18
will not be absorbed, while the distorted components, which emerge
as a result of dispersion and non-linear interactions and which do
not have the same intensity and/or amplitude as the clock pulse,
will be strongly absorbed. The distorted pulse 16 is thus restored
to the shape of the clock pulse. The restored clock pulse 20 is
comprised within the transmitted beam C. The dispersion and
non-linear interaction components are comprised within the absorbed
signal D.
[0087] FIG. 9b illustrates a coherence filter 22. This coherence
filter operates on the same principle as the pulse recovery device.
Namely, that the absorption of the components of a signal beam A
which are coherent with respect to the control beam B can be
enhanced or cancelled. The coherence of the transmitted beam C with
respect to the control beam B can thus be reduced or increased,
respectively. Again, the components not transmitted are comprised
within the absorbed signal D.
[0088] FIG. 9c illustrates an optical gate 24. The intensity of the
transmitted beam C is controlled by the phase and/or intensity of
the control beam B. The optical gate can therefore act as an AND
gate. In a first embodiment, the phase .phi. of the control beam B
is set so that a standing wave node 4 is present at the location of
the metamaterial element 2. The transmitted beam C then acts as an
output signal, which only has a high intensity if both the signal
beam A and control beam B incident on the metamaterial element 2
have that same high intensity. In a second embodiment, the input
signal of the control beam B is measured by the phase .phi., so
that a phase .phi. where a node 4 is present at the metamaterial
element 2 constitutes a high input signal where as a phase where an
antinode 6 is present at the metamaterial element 2 constitutes a
low input signal. The output signal C thus only has a high
intensity if both the signal beam A has a high intensity and the
phase .phi. of the control beam B is chosen so that it acts as a
high input signal (generating a node 4 at the metamaterial element
2). In all embodiments, the non-transmitted light in the optical
gate 24 is comprised within the absorbed signal D.
[0089] FIG. 10a illustrates an optical pulse picker/selector
device. Coherently-controlled metamaterial absorption/transparency
is employed to select individual optical pulses from an incident
signal pulse train A. Pulses will be transmitted to output C with
negligible or low loss by the selector when they are temporally
coincident on the metamaterial 2 with a control pulse in channel B
of the correct phase. Where no control pulse is present, `reject`
signal pulses will experience strong (single-beam) absorption
losses in the metamaterial. As schematically illustrated, this can
be used in a data transmitter to write a signal onto a pulse train,
thereby to encode data. Another application would be as a frequency
divider to eliminate every nth pulse from a pulse train.
[0090] FIG. 10b illustrates a frequency selector device for
wavelength division multiplexed (WDM) signals. Where the input
signal A is made up of two or more frequency components, as in WDM,
one or more of these can be selectively and simultaneously
transmitted as signal C (with negligible or low loss) by a
metamaterial absorber 2 through coherent interaction with a control
input B at the target frequency(ies). `Rejected` frequency
components (absent from the control input) will experience strong
(single-beam) absorption losses in the metamaterial. The schematic
illustration shows the form of the control beam for picking out a
single frequency.
[0091] FIG. 10c illustrates a dynamic spatial mode selector device.
Where the input signal A includes two or more spatial modes, one of
these can be selectively transmitted as signal C (with negligible
or low loss) by a metamaterial absorber 2 through coherent
interaction with a control input B with the required mode
structure. `Rejected` modes will experience strong (single-beam)
absorption losses in the metamaterial.
[0092] FIG. 11 is a schematic system of an optical fibre
transmission system incorporating the pulse recovery device of FIG.
9a. FIGS. 12a, 12b and 12c are graphs showing the spectral density
of the distorted signal, clock signal and improved signal
respectively for the system of FIG. 11, as now described. An
optical fibre telecom line 30 which forms at least part of a
transmission path from a transmitter to a receiver has an inline
metastructure film 2. The inset shows how the metastructure film
has been formed part way along a fibre transmission line by first
forming the film on an end facet of one fibre and then splicing
that fibre to a further fibre as already described with reference
to FIG. 2b. The fibre core is shown by the dashed line in the
inset. A signal, typically a data bearing signal, will be injected
into the fibre at the transmission end, at which point it will be
substantially free of distortion. As the signal propagates along
the fibre it will gradually become increasingly distorted owing to
effects such as dispersion. Distortion in the transmission line 30
can be cleaned up as described above in connection with FIG. 9a.
Namely, a clock signal can be injected into the fibre from the
receiver side so that it is incident on the metamaterial 2 in the
opposite direction than the distorted signal sent from the
transmitter. In the system schematic, the clock signal is shown
being injected into the transmission line via an intensity and
phase modulator, in the form of a variable attenuator and phase
delay 32, and a circulator 34. The modulator serves to allow the
intensity and/or phase of the clock signal (acting as the control
signal) to be adjusted so as to lock a node of the interference
pattern in the plane of the metamaterial. The desired components of
the distorted signal are thus transmitted substantially without
loss, whereas the undesired components of the distorted signal,
i.e. the distortion, are partially or substantially fully
suppressed. The schematic graphs of distorted signal, clock signal
and improved signal are shown in the frequency domain as spectral
density v frequency, but could also be plotted in the time domain
as power v time. The restored signal is then routed by the
circulator 34 to the receiver. A circulator is shown for
convenience of illustration, but it will be understood that any
equivalent routing element could be used.
[0093] Further embodiments may additionally provide for modulation
of the absorptive properties of the metamaterial element itself or
a layer of another material arranged with the metamaterial element,
for example a layer of another material formed on the other surface
of a common substrate. The controllable absorptive properties that
may be exploited may be controllable via temperature, applied
voltage or current, or a further light beam, for example. Further
background for gold, carbon nanotubes, chalcogenide glass and
reconfigurable photonic metamaterials may be found in the following
references respectively: [0094] M. Ren, B. Jia, J. Y. Ou, E. Plum,
J. Zhang, K. F. MacDonald, A. E. Nikolaenko, J. Xu, M. Gu, N. I.
Zheludev "Nanostructured plasmonic medium for terahertz bandwidth
all-optical switching" Adv. Mater. 23, 5540 (2011) [0095] E.
Nikolaenko, F. De Angelis, S. A. Boden, N. Papasimakis, P. Ashburn,
E. Di
[0096] Fabrizio, and N. I. Zheludev "Carbon nanotubes in a photonic
metamaterial" Phys. Rev. Lett. 104, 153902 (2010) [0097] Z. L.
Samson, K. F. MacDonald, F. De Angelis, B. Gholipour, K. Knight,
C.-C. Huang, E. Di Fabrizio, D. W. Hewak, and N. I. Zheludev
"Metamaterial electro-optic switch of nanoscale thickness" Appl.
Phys. Lett. 96, 143105 (2010) [0098] J. Y. Ou, E. Plum, L. Jiang,
and N. I. Zheludev "Reconfigurable photonic metamaterials" Nano
Lett. 11 (5), 2142-2144 (2011)
[0099] Optically responsive effects can be achieved with carbon
nanotubes, phase change materials, such as chalcogenide glasses
(including GeSbTe (GST) compounds and GaLaS (GLS) compounds) and
through an optical nonlinearity of the metamaterial element itself,
such as two photon absorption. Thermally responsive effects can be
achieved with mechanically reconfigurable structures and liquid
crystals. Electrically responsive effects can be acheived with
phase change materials, such as chalcogenide glasses (including
GeSbTe (GST) compounds and GaLaS (GLS) compounds) and liquid
crystals. References to a reversible phase change refer to changes
between a glass and a crystalline phase.
[0100] For convenience we provide a look up table below showing the
thickness in nanometres of the metamaterial element for a number of
different operating wavelengths A in nanometres in the visible to
near infrared region.
TABLE-US-00001 Thickness/Periodicity (nm) .lamda. (nm) 1/3 1/4 1/5
1/6 1/7 1/8 1/9 1/10 1/11 1/12 1/13 1/14 1/15 400 133 100 80 67 57
50 44 40 36 33 31 29 27 500 167 125 100 83 71 63 56 50 45 42 38 36
33 600 200 150 120 100 86 75 67 60 55 50 46 43 40 700 233 175 140
117 100 88 78 70 64 58 54 50 47 800 267 200 160 133 114 100 89 80
73 67 62 57 53 900 300 225 180 150 129 113 100 90 82 75 69 64 60
1000 333 250 200 167 143 125 111 100 91 83 77 71 67 1100 367 275
220 183 157 138 122 110 100 92 85 79 73 1200 400 300 240 200 171
150 133 120 109 100 92 86 80 1300 433 325 260 217 186 163 144 130
118 108 100 93 87 1400 467 350 280 233 200 175 156 140 127 117 108
100 93 1500 500 375 300 250 214 188 167 150 136 125 115 107 100
1600 533 400 320 267 229 200 178 160 145 133 123 114 107 1700 567
425 340 283 243 213 189 170 155 142 131 121 113 1800 600 450 360
300 257 225 200 180 164 150 138 129 120 1900 633 475 380 317 271
238 211 190 173 158 146 136 127 2000 667 500 400 333 286 250 222
200 182 167 154 143 133
[0101] For example, if the metamaterial element is to be a tenth of
a wavelength in thickness at an operating wavelength of 1500 nm,
then it would be 150 nm thick. The same table can be used to
consider the periodicity of the metastructuring, so for example if
the periodicity of the metastructure was intended to be a third of
a wavelength at 900 nm, then it would have a periodicity of 300 nm.
We reserve the right to claim any finite combination of dimensions
from the above table, in particular we reserve the right to claim
any of the above values for the upper thickness of the metamaterial
element in a specified range of the form "the metamaterial element
has a thickness of less than . . . ", and an upper periodicity for
the metastructuring of the metamaterial element of the form "the
metamaterial element has a metastructure periodicity of less than .
. . ".
[0102] It is noted that in the system example a single laser source
is used to generate both the control and signal beams through the
use of a beam splitter. In other embodiments, separate laser
sources could be used to generate the respective beams. For example
in a telecoms application, the signal beam source could be on the
transmitter side and the control beam source could be on the
receiver side.
[0103] For optimal performance of the device when operating in
phase modulation mode, i.e. the device is controlled by phase
modulation of the control beam, it is preferable that the intensity
of the signal and control beams have similar values. For optimal
performance of the device when operating in intensity modulation
mode, i.e. the device is controlled by intensity modulation of the
control beam, it is preferably that the control beam intensity is
modulated between an intensity similar to the intensity of the
signal beam and a zero intensity value.
[0104] It is further noted that performance in some applications
can be improved by cascading the devices. For example, in the
device applications which serve to remove distortion or noise from
a signal, such as the clock recovery application and coherency
filter application, it may be beneficial to arrange 2, 3 or more
metamaterial elements in series, or to route the signal through the
same metamaterial element 2, 3 or more times.
[0105] In summary, the light-by-light modulation as presented
provides functionality for analogue and digital modulation and
switching without the need for intense laser fields or an optically
non-linear medium, as has previously been the case. This
modulation/switching functionality can therefore be delivered at
extremely low power levels. As described above, the light-by-light
modulation described herein can provide devices with an extremely
high, terahertz frequency modulation bandwidth that is determined
by the width of the resonance in the metamaterial element. Using
plasmonic metal or metallic structures, light-by-light modulators
can be realised throughout the visible and near-infrared parts of
the electromagnetic spectrum, where plasmonic resonances can be
engineered and metal Joule losses are substantial. A metamaterial
element of the type normally associated with being exploited as a
non-linear medium for hosting effects such as four wave mixing or
two-photon absorption is instead being incorporated in a device and
exploited for simple resonant absorption which is a linear effect
which therefore has no intensity threshold to overcome for it to
operate. Extremely low power and rapid modulation is therefore
possible.
* * * * *